Will Lack of Nitrogen Limit the Ability of Earth's Forests, Shrublands and Grasslands to Slow the Rate of Rise of the Air's CO2 Content?Volume 6, Number 50: 10 December 2003

In the opening paragraph of a Carnegie Institution press release dated 27 Nov 2003, Dr. Christopher Field, Director of the Institution's Department of Global Ecology, says "we should not count on carbon storage by land ecosystems to make a massive contribution to slowing climate change," noting further that "lower storage of carbon in these ecosystems results in a faster increase in atmospheric carbon dioxide, leading to more rapid global warming." Is this chain of claims correct?

Support for the strident statements is said to reside in the Science magazine report of Hungate et al. (2003), which according to the Carnegie press release "shows that the availability of nitrogen, in forms usable by plants, will probably be too low for large increases in carbon storage." In reading the Science report, however, one finds the claimed support comes not from any new experimental evidence (not a single reference from 2003 is cited), but from a brief and rather selective review of somewhat older scientific papers, which review thus overlooks a number of more current studies whose findings run counter to the contentions of Hungate et al.

Interestingly, Hungate et al. begin their polemic by noting a few facts that actually argue against their claims. They report, for example, that "tree C:N increases with atmospheric CO2 concentration (Hungate, 1999; Rastetter et al., 1992)," which clearly indicates that increasingly more carbon can be stored in tree tissues per unit of nitrogen stored therein as the air's CO2 content rises. In addition, they note that "soil C:N could also increase with rising atmospheric CO2 concentration, allowing soil carbon accumulation without additional nitrogen [our italics]."

Nevertheless, and in spite of their open acknowledgment that the storage of a given amount of carbon in tree tissues and soils in a high-CO2 world will not require as much nitrogen as it does today, Hungate et al. go on to claim that the future availability of nitrogen will likely still be too low to support large increases in terrestrial carbon storage, primarily because of their contention that "when CO2 enrichment increases soil C:N, decomposing microorganisms require more nitrogen," and that "this effect can reduce nitrogen mineralization," which they say is "the main source of nitrogen for plants," citing as supportive evidence for this line of reasoning the study of Gill et al. (2002). As we demonstrate in our Editorial of 22 May 2002, however, the study of Gill et al. is by no means the final word on this important subject, nor does it provide compelling evidence for what Hungate et al. attempt to wring out of it. In fact, it actually provides evidence for just the opposite conclusion, as we indicate in our editorial.

So where can we turn to gain further enlightenment on this issue? One good source is our own CO2 Science Magazine, where we have reviewed several pertinent studies, all of them published in 2003, that provide a vastly different perspective on the issue from the one that Hungate et al. are attempting to promote. Doing so, we find the following.

Finzi and Schlesinger (2003) measured and analyzed the pool sizes and fluxes of inorganic and organic N in the forest floor and top 30 cm of mineral soil during the first five years of differential atmospheric CO2 treatment in the ongoing Duke Forest FACE study, where half of the experimental plots are maintained at a mean CO2 concentration 200 ppm above ambient. They found that the presence of the extra CO2 "significantly increased the input of C and N to the forest floor and the mineral soil." However, they say "there was no statistically significant change in the cycling rate of N derived from soil organic matter under elevated CO2" and that "neither the rate of net N mineralization nor gross 15NH4+ dynamics were significantly altered by elevated CO2." In fact, they could find "no statistically significant difference in the concentration or net flux of organic and inorganic N in the forest floor and top 30-cm of mineral soil after 5 years of CO2 fumigation," adding, in direct contradiction of the claim of Hungate et al., that "microbial biomass was not a larger sink for N." Based on these findings, they actually rejected their own original hypothesis (essentially the same as the hypothesis of Hungate et al.) that the extra CO2 provided to their experimental plots would significantly increase the rate of nitrogen immobilization by the soil microbial communities found within the CO2-enriched FACE arrays.

At the same Duke Forest FACE site, Schafer et al. (2003) measured net ecosystem exchange (NEE) and net ecosystem production (NEP) during the third and fourth years of the long-term CO2 enrichment study being conducted there. They found that the extra 200 ppm of CO2 supplied to the CO2-enriched FACE arrays increased the entire canopy's net uptake of CO2 (NEE) by fully 41%, and that canopy NEP was increased by 44% (which corresponds to an NEP increase of roughly 66% for the more commonly-employed atmospheric CO2 enhancement of 300 ppm). In addition, they found that 87% of the extra NEP "was sequestered in a moderately long-term C pool in wood." This large increase in solidly-sequestered carbon is all the more amazing in light of the declaration of Finzi and Schlesinger that the soil at the Duke Forest FACE site is in "a state of acute nutrient deficiency." Schafer et al.'s experimental data thus fly in the face of the press-release-reported claim of Jeffrey Dukes, second author of the Hungate et al. study, that "plants will need more nitrogen if they're going to lock up more carbon."

In another approach to determining the need, or lack thereof, for large amounts of extra nitrogen to sustain future CO2-induced increases in forest productivity, Cannell and Thornley (2003) conducted a number of analyses with the Edinburgh Forest Model. Their investigation revealed that although net primary production (NPP) may depend on soil organic matter and specific nitrogen mineralization rate initially, in the long term "NPP is determined by climate rather than soil properties such as soil organic matter and specific mineralization rate." We also note, in this regard, that as used in this sentence, the word climate could well be replaced with the words "all non-edaphic environmental factors that promote plant growth," which would include the air's CO2 concentration in addition to its meteorological parameters; for as Cannell and Thornley ultimately conclude, given enough time, properly "modeled ecosystems tend to generate amounts of soil organic matter that are able to supply nitrogen at rates which do not greatly limit plant growth."

What is being found to be the case for forests, in this regard, is also being found to be the case for grasslands. Richter et al. (2003), for example, hypothesized "that increased below-ground translocation of photoassimilates at elevated pCO2 would lead to an increase in immobilization of N due to an excess supply of energy to the roots and rhizosphere," which hypothesis is readily recognized as being the contention of Hungate et al. To test this hypothesis, Richter et al. measured gross rates of N mineralization, NH4+ consumption and N immobilization in soils upon which monocultures of Lolium perenne and Trifolium repens had been exposed to ambient (360 ppm) and elevated (600 ppm) concentrations of atmospheric CO2 in the Swiss FACE study near Zurich. Their findings? They report that after seven long years of exposure to elevated CO2, "gross mineralization, NH4+ consumption and N immobilization in both the L. perenne and the T. repens swards did not show significant differences." And once again, in direct contradiction of the claim of Hungate et al., they report that the size of the microbial N pool and immobilization of applied mineral 15N were not significantly affected by elevated CO2. Hence, like Finzi and Schlesinger, they too openly acknowledge that "the results of this study did not support the initial hypothesis [our italics]."

Also out of Switzerland comes the revealing report of Thurig et al. (2003), who studied what they describe as a nutrient-poor low-productivity calcareous grassland for a period of five years via the screen-aided CO2 control (SACC) technology of Leadley et al. (1997). At the end of this period of plant exposure to atmospheric CO2 concentrations of 360 and 660 ppm, they were still finding that "the effect of elevated CO2 on the number of flowering shoots (+24%) and seeds (+29%) at the community level was similar to above ground biomass response," which is a sizeable response for a nutrient-poor low-productivity grassland.

On another front, Hungate et al. estimate biological nitrogen fixation "to increase linearly by 10% (low) or 45% (high) with CO2 doubling," yet significantly greater responses have been observed. As part of a long-term grassland FACE experiment, for example, Lee et al. (2003) grew the N2-fixing legume Lupinus perennis L. in nine-species plots in ambient air or air enriched with CO2 to a concentration of 560 ppm, finding that the proportion of Lupinus N derived from symbiotic N2 fixation increased from 43% to 54%, which combined with the observed CO2-induced increases in plant biomass production resulted, in their words, "in a doubling of N fixed per plot." And that was for only a 50% increase in the air's CO2 content! Consequently, in a CO2-enriched world of the future, legumes like Lupinus will likely provide the ecosystems within which they occur with considerably more nitrogen than is currently available to them.

One final way in which the deep-rooted woody plants of earth's arid and semi-arid ecosystems may acquire more nitrogen as they respond to the ongoing rise in the air's CO2 content will be by simply bringing it up from greater soil depths. Walvoord et al. (2003), for example, have recently discovered what they describe as "a large reservoir of bioavailable nitrogen (up to ~104 kilograms of nitrogen per hectare, as nitrate) [that] has been previously overlooked in studies of global nitrogen distribution." This amount of new nitrogen, they say, "raises estimates of vadose-zone nitrogen inventories by 14 to 71% for warm deserts and arid shrublands worldwide." In commenting on this newly-found store of nitrogen, Stokstad (2003) calls it a "potential bonanza" and says that Duke University ecologist Robert Jackson actually "wonders if the pool of nitrate could help explain why deep-rooted woody plants have invaded the Southwest over the past century or so."

In light of these several up-to-date real-world observations and experimental findings, it would appear that the contentions of Hungate et al. -- which but only a year ago would have been received as accepted wisdom -- no longer resonate with reality. Consequently, just the opposite of what they conclude is likely to be the case, i.e., we can "count on carbon storage by land ecosystems to make a massive contribution to slowing climate change."